Process for improving the magnetic properties of silicon steel



April 6, F PAWLEK ET AL PROCESS FOR IMPROVING THE MAGNETIC PROPERTIES OF SILICON STEEL Filed Feb. 26, 1955 Fig. l.

HELONGATION NOT GREATER THAN l0l2% Attovneg.

Patented Apr. 6, 1937 UNITED STATES PROCESS FOR. IIVIPROVING THE MAGNETIC PROPERTIES OF SILICON STEEL Franz Pawlek, Berlin-Niederschoneweide, and

Otto Dahl, Berlin-Friedenau, Germany, assignors to General Electric Company,.a cor-- A v poration of New York v Application February 26, 1935, Serial No. 8,27

ln Germany May 14, 1934 3 claims.' (C1. 14s-12) The present invention relates to magnetic material and more particularly to a method for improving the magnetic properties of such material. v

l5 It is one of the objects of the invention to provide an improved magnetic material having a large grain size. A further object .of the invention is the provision of means whereby the final cold reduction vof large grain size magnetic material may be varied Within comparatively wide limits without causing substantial change in grain size.

The novel features which are characteristic of our invention are set forth with particularity in 16 the appended claims. The invention itself however will best be understood by reference to the following specification and appended claims when considered in connection with the accompanying -drawing in which Fig. 1 is a recrystallization diafluencing the size of the grain at will seemed clearly determined by previous experiments. For

all metals and alloys there is obtained in principle the same recrystallization diagram as shownin Figure 1. This gure as hereinbefore set forth represents a section from the space diagram: size of grain-degree of deformation-for a constantV recrystallization temperature. A maximum grain E size is obtained by acertain degree of deformation which occurs with small reductions in thickness, the so-called critical degree of stretch or elongation.

This critical degree of elongation is a result of the inuenceof cold elongation upon the crystallization property. In-this case it is best to distinguish 'between the recrystallization capacity and recrystallization nuclei. Both are first brought about by cold deformation, each being enlarged or multiplied by increasing cold deformation. The critical elongation or 'stretching degree at which the largest grain is uniformly obtained in the whole work-piece is characterized by individual newly-growing grainsl below the critical elongation no uniform recrystallization of the Work-piece is possible since the deformation necessary for obtaining the recrystallization property is not exceeded in all parts of the work-piece. Therefore no uniform grain is found here. Regions with large new crystals alternate With regions of small initial crystallites which are probably still unchanged. In Figure 1,

grain size, stretching degree, recrystallization temperature, derived from the tracing of such curves for different temperatures, applies under all conditions, according to all previous investigations. According to the size of the initial crystals or the nature of the material thev critical stretching degree may vary to some extent, but

cases are not known where it amounts to more than 10-12%, and in all cases it is a factthat to obtain a grain of the largest possible size the critical elongationof stretching degree must be observed` as strictly as possible (up to about 16% reduction in thickness).

"The knowledge of this recrystallization diagram has been of the greatest importance fon metal working. On the one hand, it enables us to avoid operations which produce a coarse struc,- ture which`may be harmful in the case ofstructural materials subjected to compressive stress.

On the other hand, it indicates the manner of l obtaining a coarse structure in an ideal case, even of a mono-crystalline work-piece'. If the great scientific interest in mono-crystals is disregarded, the main interest in such enlargement of the grains is in connection with magnetic materials. In this case it was recognized that with the increase in size of grain a considerable improvey ment occurs, i. e. a decrease-in the coercive force and hysteresis losses and an increase in the permeability. For such improvements the methods prescribed by the recrystallization diagram were intentionally used. The dimculty in their application was that the reduction of thickness to be eiected wassmall and had to be maintained very accurately,.since any upward or downward variation caused a disturbance of the coarse Icrystallization either a general considerable decrease in size or incompletely recrystallized regionsy that 'l tallic materials is obtained, consists insubjecting such materials to cold deformation above the critical degree of elongation. before final annealing. This cold deformation should be greater than 15%. The most favorable degree of elongation also depends upon the space lattice or the composition of the material. In the case of silicon steel the range between about 20 and about 40% has proved to be particularly suitable. This last cold deformation is followed by a nal anneal at elevated temperatures which are preferably higher than 950 C. and are chosen preferably in the range between 950 C. and 1100 C. when using silicon steel and in general arel at least 300 C. above recrystallization temperature.

40 By means of such a treatment considerable grain enlargement and thus improvement in the magnetic properties of the treated materials are obtained, the grain enlargement being greater than that obtained by using the critical stretch- 45 ing degree. The process according to the present invention has the further advantage that the nal degree -of elongation can be varied within comparatively wide limits without a substantial change occurring in the grain enlargements ob- 50 tained, a. fact which greatly facilitates and reduces the cost of manufacture.

In addition to the nal elongation and the nal annealing, the preceding stages of treat- V ment applied to the material also have a. certain y55 inuence on the size of grain obtained. Thus,

it is preferable to choose for the preliminary 'elongation a cold deformation which is not too small.

Itis preferable not to ix the preliminary elongation below a 15% reduction in thickness. An upper limit does not exist in the case of the preg cngation but the grain enlargement can be increased within certain ninas by increasing the preliminary elongation. 'I'he temperature of the anneal between the preliminary .elongation and the inal elongation is preferably chosen not too high and for best results should be as little as possible above e recrystallization temperature and at most not more than 400 C. above the recrystallization temperature. .By preliminary annealing at excessive temperature the total result may becomedoubtful since thereby a coarse grain is produced which at this stage.

In Figure 2 the dependence of the size of the grain upon the nal elongation` for silicon steel is unfavorable (2.8% Si) is represented-for three different 'preliminary annealing temperatures. The alloy was the individualwresults shown 'the following are' kept constant: (l) the last recrystallization temperature and time: 4 h. 1025a C.; (2) the next to the last cold elongation 75%. The following are varied: (l) the annealing temperature employed before the last cold stretching: 750 C., 850 C. and 950 C.; (2) the final elongation. 'I'he result is plotted in accordance with the curve referred to at the beginning. All the curves at rst take the hitherto well-known course: considerable Icoarseness of the grain being obtained by a low critical degree of elongation then a decrease of the size of the grain. In the case of the specimen previously annealed at 950 C. this course remains practically unchanged up to the highest degree of elongation. In the case of the specimens previously annealed at 850 C. and '750 C. With about 20% elongation, on the other hand, there occurs a sudden rise of the curve to very great dimensions of the grain. This size of the grain is maintained over a comparatively large range of elongation and then falls again very quickly to smalldimensions. The course of the'curves is, of course, diierent for different materials. It is also possible in the case of one and the samel material to choose differentj individual stages of treatment in accordance with the actual circumstances, for example, in one case the eiect of the preliminary degree of elongation on the sizeof the grain may be employed and in the other case the effect of the preliminary annealing temperature may be employed. It can easily be ascertained from experiments which of the numerous variants available are the most favorable in each case. 'I'he treatment should in any case be chosen in such a mannerthat the recrystallization structure favorable for the abnormal growth is obtained.

0n the basis of previous tests `this can be ob' having a tlne structure'showed for the latter in the case of a size of grain of 0.0056 mm.2 a watt loss .of 1.5-1.6 W/kg., while the former in the case of a size of grain of 3850 mrn.2 showed a 'watt loss of only 0.7-0.9 W/kg.

The use of they process according to the inven- Ytion for obtaining mono-crystals or a coarse macro-crystalline structure, however, is not limited to the iron-silicon alloys described, but can also be used for other materials in the same manner. The degree of elongation and annealing temperatures actually most favorable can be determined without diiliicultyv in each case by tests. What we claim as new and desire to secure by Letters Patent of the United States is:

1. The process for improving thek magnetic properties of silicon steel which comprises cold rolling the steel to' eiect a reduction in thickness of more than 15%, annealing the steel at a tem? perature at which recrystallization takes place but below 950 C., cold rolling the steel to eifect a reduction of 15% to 45% in thickness and finally annealing the steel at a temperature between 950 C.,and 1100 C., said rst mentioned anneal and last reduction being such that the product after said nal anneal exhibits a structure of large macro-crystals.

2. The process for improving the magnetic properties of silicon steel which comprises 4cold rolling the steel to eiect a reduction in thickness of more than 15%, annealing the steel at a teml perature between about '150 C. and 850 C., cold rolling the steel to eiect a reduction of about 20% to 40% and finally annealing the steel at a temperature between 950 C. and 1100 C.

3. The process for improving the magnetic properties of silicon vsteel containing about 3% silicon which comprises cold rolling the steel to effect a reduction of 75% in thickness, annealing'- at a temperature between 750 C. and 850 C., cold rolling the steel to eiect a reduction of about 20% to 40% in thickness and annealing the 10 steel at a temperature in the neighborhood of 

